Sensitivity control system for automobile radio receivers



June 21, 1938. 2,121,427

SENSITIVITY CONTROL SYSTEM FOR AUTOMOBILE RADIO RECEIVERS L. T. FOWLER Filed April 50, 1957 Patented June 21, 1938 N D A E ware PATENT orrior Application April so, 1937, SerialNo. 140,064

10 Claims. (01. 25o 20)' "The present invention reIa'tes to a sensitivity control system for automobile radio receivers and thelikegwhich are normally subjected to ignition and otherinterferencaand hasfor its object to provide an improved sensitivity control system of the character referred towhich permits of an extended control circuit fromthe receiving apparatus and which does not introduce interference signals into the receiving apparatus.

Sin'ce'the sensitivity control for automobile receivers and the like must be extended to the instrument panelof the automobile, the control circuits involves additional filters and shielding to prevent picking up ignition and other interference.

Cathode resistor control of the intermediate frequency amplifier tubes alone is unsatisfactory, although less sensitive to interference. Control of the intermediate-frequency 'amplifier'tube or tubes requires biasing to such a'wi'cle degree that the tubeslose sufficientlpower to *drive the diode rectifier-or second detector. Also, with this method of control, the distortion increases and additional second harmonic signals are generatedin the plate circuit of' the intermediate frequency amplifier tube ortubesa This causes the beat note condition to be worse than normal, so that addi tional filtering is necessary to suppressthe beat notes. I

It is, therefore, a further object of the present invention to provide a sensitivity control system foruautomobile radio receivers and the like which effectively overcomes substantially all of the above disadvantagesrand which'is at the same tim easily p'rovidedat low cost.

In a radio. receiving "system, the automatic volume control circuit is provided with suitable tion to provide a sensitivity controlsystem for automobileradio receivers and thellke having a filtered automatic volume control circuit which is applied to the return circuit of the automatic control means to take advantage of the inherent filtering and without resulting in any control '6 with asecond detector and amplifier tube 1.

connection with the cathode circuits of any of the amplifier tubes, whereby interference pick-up is reduced to a minimum. i

The invention will, however, be better understood from the 'following description,when con- 5 sidered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims, v

Inthe drawing, Figure 1 is a schematic circuit diagram of a portion of a'radio receiver for automobiles and the like provided with a sensitivityv control system embodyingthe invention, and

Figure 2 is asimilar schematic circuit diagram, being a portion of the circuit of Fig. 1 showing a modification of the sensitivity control circuit.

Referring to Fig. 1, 5 is an intermediate frequency amplifier tube which iscoupled through an intermediateifrequency coupling transformer The latter is of the double-diode, triode type, having a cathode 8, a detector diode anode 9 connected with the tuned secondary circuit iii of v the transformer 6, an automatic volume control diode anode ll connected with the tuned primary circuit |2 of the transformer 6 through a coupling capacitor I3, and havinga control grid l4 and an output anode 15 for the triode amplifier portion thereof." The cathode B is connected to chassis or ground I 6, which isalso the automatic volume control return circuit, through a self bias or cathode resistor comprising three sections l1, l8

through a diode output resistor 2| and a shunt 5 Volume. control resistor 22,conneced therewith through a coupling capacito'r 23. The grid It receives a biasing potential from a tap 24 between the resistor sections l8 and I9, to which tap point the volume control resistor 22 is connected at its low potentialend, asshown. i

' The output anode l5' o'f the triode section is coupled" through" suitable resistance coupling means 25 with a first audio frequency amplifier v tube26, in t e output anode circuit 21 of'which an output coupling device 28 is provided for connection with additional amplifier stages or other output devices-(not shown). The amplifier stage 26 isprovided witha tone control means comprising a capacitorfSU connected to the output circuit 21 and having a control lead 3| connected through a lead 32 in an extension cable 33, with atone control resistor 34 adapted for mounting on the instrument panel of the automobile. A ground lead35connected with the chassis I6 is extended through a lead 36 in the cable 33 to a manually adjustable tone control contact 31 in connection with the resistor 34, whereby the tone is controllable in a well known manner, the resistance in circuit with the capacitor 30 being ireduced to reduce the high frequency response of the amplifier by the increased shunting action of the capacitor across the signal circuit.

The automatic volume control circuit is connected with the diode anode II, which receives signals from the primary of the transformer B and is thereby more broadly responsive than the secondary. The diode circuit is provided by a diode output resistor 40 connected between the diode anode H and the terminal 24 or other suitable tap along the cathode resistor. This places a delay potential on the diode anode ll provided by the drop through the resistors 11 and I8. It will be noted that a lesser delay potential is placed on the diode anode 9 for the detector through the connection of the detector output resistor 2| with a tap point 4| between the resistor sections I1 and i8.

The automatic volume control potentials developed across the resistor 40 are applied to an automatic volume control lead "for the preceding amplifier tubes, through a series filter resistor 43 and a second filter resistor 44, for which bypass capacitors 4545 are provided. v

' The controlled amplifier tubes, represented by the intermediate frequency amplifier tube 5, are

connected with the automatic volume control lead 42 through suitable leads indicated at 46, and 41. In the present example, the control grid 48 of the amplifier 5 is connected through the input circuit 49 with the lead 46, While the cathode 50 is connected to chassis or ground 5! through a self bias resistor 52. Other-amplifier tubes (not shown) are'connected in a similar manner to the chassis and automatic .yolume 'control' lead 41, whereby the chassis provides the automatic volume control return circuit connection.

Intermediate frequency signals are applied to the second detectorand automatic Volume controlmeans included in the tube land associated ,circu'its, through the amplifier} from an intermediate frequency input transformer 53, the secondary of which is included'in the input circuit 49 and the primary 54 of which may be included in the anode output circuit of either the first detector or apreceding intermediate frequency amplifier tube (not shown), as is well understood. I

The automatic volume control lead 41 may be connected with any of the preceding gain control tubes as normallyused in a receiver such as the usual radio frequency amplifier, first detector, and other intermediate frequency amplifier tubes, the connections being asshown for the tube 5. In this circuit, the biasingpotential between grid and cathode'is provided-by the automatic volume control diode output resistor 40 and thecathode resistor [9 as two potential sources, togetherwith the potential source provided by 'the self bias resistor 52 at the amplifier tube. The polarity 'of these sources is indicated adjacent thereto.

The bias circuit arrangement is such that the automatic volume control diode resistor 40, instead of being returned directly to ground or chassis I6, is returned thereto through the potential source l9 whichis positive at the terminal 24 with respect to ground or chassis by an amount sistance for such self bias use by an amount such that the cathodes of the controlled tubes, such as the cathode 50, may have a positive potential with respect to ground or chassis sufiicient to provide the grids, such as the grid 48, with a normal negative bias in the absence of signals,

- whereby maximum sensitivity is obtained. The

their cathodes by the amount of the potential 4 provided by the resistor I9 and the over-all sensitivity of the receiver-will be reduced accordingly, to some desired lower value, suitable, for example, for receiving local signals without' interference. This control maybe efiected by extending a. lead 60 from the terminal 24, through a cable lead 6| in the cable 33. to a switch 52 representing any suitable control device for grounding the lead 6|. In the present case, the switch serves to connect the lead 6! to ground through the leads 36 and 35.

Because of the connection provided with the ground return circuit of the automatic volume control system as shown, interference. from ignition ,orother sources introducedinto the extension leads 36 and 6! have no effect upon the receiving circuits because of the filtering in the automatic volume control circuit, and, the fact that the controlled potential source is connected adjacent to ground;

Theextensible control circuit shown may be connected with the receiver through terminals indicated at 63 and the control leads may in certain, cases be grounded-at R. F. and I. F. potentials to the cable shield 33 and ground or chassis 64 andg65 through suitable high frequency by-pass capacitors 66 and 61 at the receiver terminals. The by-pass connections at this point are effective in' removing ultrahigh frequency disturbances from the internal circuits of the receiver and in further isolating the same electrically' from the external-circuits connected therewith: 1 I Y i The circuit shown has been found to be satisfactory in connection with automobile receivers operating in the presence of heavy ignition and other interference potentials. Furthermore, the two-point sensitivity control is effective to provide what may be termed local and distance receiving conditions; depending upon the particular tubes used, andthe amount of potential change 'required, for reducing the sensitivity for local receiving conditions.

-The range of control is determined 'by-the potential established. across the auxiliary or sensitivity control resistor l9 provided asthe fixed potential source in the automatic volume control circuit return lead to ground or chassis.

In case, as is desirable, the fixed potential source for the sensitivity control is provided as'a part of the cathode resistor of the combined second detector and amplifier-tube, the resistance of the resistor section I 9 should be relatively'low with respect to the resistance of the remainder of the resistor comprising sections i7 and I8, in order that as the sensitivity control section is short-circuitedithe change in the bias on the receiver.

If variable control of the sensitivity is desired, the automatic volumecontrol lead to the source at the terminal 2'4 may be made variable, or the section I9 maybe extended externally of the receiver and avariable return connection may be made therewith as shownin' Fig. 2, to which attention is now directed, and in which like circuits 'and circuit elements as in Fig. 1 are designated by the same reference numerals.

Inthis case, the terminal 24 and the ground [6 are connected through the leads GI] and 35 with the cableleads 6| and 36, respectively, as beiore, whichterminate in a potentiometer resistor 10 having a movable contact H connected through a cable lead 12 with a leadl3 in the receiver for the automatic volume control ground return from the automatic volume control diode output resistor 40, whereby the said lead is variably connected with the potential source l9. It will be seenthat movement of the potentiometer contact H permits a varying potential tobe applied to the automatic volume control circuit in opposition to the higher than normal biasing potential provided in connection witheachamplifier tube by the self bias grounded cathode resistors, thereby permitting the sensitivity to be reduced gradually froma maximum to aminimum determined by the range of' potential varia tion available across the source l9. In certain cases, where interference is less=severe, the resistor l9 may be omitted and the resistor 'Hlmay provide the potential droprequired-for-the range of sensitivity control. 1

In the present case, the tone control'connection is omitted, although it "may be' "applied through the cable connection in the same manner r of local signals.

as shown in the circuit of Fig. 1. The present circuit, however, reduces the number of leads required in the'cable which is extended to anysuitable location for the sensitivity control device.

In either of the embodiments shown in Figs. 1 and 2, the operation of the automatic volume control means is not impaired, since'the potentials set up across'the diode output resistor 40, in the presence of signals, serve further to increase the negative bias oi the controlled tubes, and therefore reduce the gain Whether the sensitivity is' set for maximum or minimum or any intermediate position by eitherof the means shown in Figs. 1 and 2. i

In either circuit, the sensitivity control' is applied in the ground return connection for the automatic volume control circuit, the latter being provided with suificient fixed negative biasing potential on the controlled tubes to overcome a seriesfixed opposing positive biasing potential of a value representing the range 1 of biasing control desired for changing the 'receiving system from a condition of maximum sensitivity to a condition of reduced sensitivityfor the reception With this arrangement, the series automatic volume control source" of variable potential may operate effectively to control the gainloc'allyunder-any conditio'nof sensitivityand theexternal circuits from the receiver are eifectively' isolated from the receiving circuits through the normal filter provided inthe automatic volume control circuit. i

I claim as my invention: V L 'I'he combination with an automobile radio receiving systemhaving automatic volume con' trol means, of an extensible sensitivity control circuit for said system comprising a chassis ground lead and a ground return lead for said automatic volume control means, means providing a controllable connection between said leads at a point remote from said receiving system to control the sensitivity between predetermined limits, and means providing a source of potential between said leads in circuit with said automatic volume control means and in opposition to the biasingpotential provided by said automatic volume control means in response to signals, the potential of said source being of a predetermined value determining the'range of sensitivity control ef fectiveat said remote control point.-

2. The combination with an automobile radio receiving system having automatic volume control means, of a remote control system having an extensible control circuit comprising a chassis ground lead, a tone control circuit including said lead,'and a ground return lead for the automatic volume control system, means for connecting said leads at a point remote from said receiving system to control the sensitivity and tone between predetermined limits, and means providing a source of potential between said ground and ground return leads in opposition to a normal biasing potential provided by said automatic volume control means and of a predetermined value determining the range of sensitivity control effective at said remote control point.

3. The combination with an automobile radio receiving system having automatic volume control means, of an extensible sensitivity control circuit for said system comprising a chassis ground leadand a ground return lead for said automatic volume control means, means providing a controllable connection between said leads at a point remote from said receiving system to control the sensitivity between predetermined limits, means providing a sourceof potential between said leads in circuit with said automatic volume control means and in opposition to biasing potential provided by said automatic volume control means in response to signals, the potential of said source being of a predetermined value determining the range of sensitivity control effective at said remote control point,said automatic volume control means including a diode signal rectifier having an output resistor and a plurality of signal amplifier tubes in said receiving system, each of said tubes having a control grid connected with the negativeend of said output resistor, a filter in said connection, and said ground return lead for the automatic volume controlsystem being'connected with the opposite end o'fsaid output resistor.

4. The combination with an automobile radio receiving system having automatic volume control means, of a sensitivity control system having an extensible control circuit comprising a chassis of signal amplifier tubes in said receiving system, each of said tubes having a self bias resistor connected with said chassis ground providing a potential in excess of the potential of said source by an amount to provide maximum sensitivity in the absence of received signals, each of said tubes having a control gridconnected with the negative end of said output resistor, a filter in said connection, and said ground return lead for the automatic volume control system being connected with the opposite end of said output resistor.

5. The combination with a radio receiver having an automatic volume control circuit provided with a grounded chassis return connection, of means providing a fixed source of positive biasing potential for said circuit in said ground re: turn connection of a potential value corresponding to a desired change in sensitivity between a condition of maximum and minimum sensitivity, means in said automatic volume control circuit providing a negative biasing potential in excess of and in opposition to said first named potential by an amount sufiicient to provide a predetermined maximum sensitivity, and means connected with said first named potential source for reducing said first named potential thereby to decrease the sensitivity of the receiving system from maximum, an extensible control circuit for said last named means connected with the automatic volume control circuit, and filter means in said automatic volume control circuit for isolating said extensible circuit from the, receiving system.

6. In a radio receiving system, the combination with automatic volume control means and a plurality of amplifier tubes controllable, thereby, of an automatic volume control circuit comprising means providing a control potential responsive to variations in received signal strength, means providing a return lead to aichassis ground for said circuit, means in said lead providing a fixed source of biasing potential for said tubes, means providing a second fixed source of biasing potential for said tubes,-the magnitude of which potential is in excess of the potential provided by said first named source and in opposition thereto by an amount to provide a condition of maximum sensitivity for said receiving :system, sensitivity control leads extending from-said first named source of biasing potential, and means connected with said leads for connecting said automatic volume control return lead'to said chassis ground.

'7. In a radio receiving system, the combination with automatic volume control means and a plurality of amplifier tubes controllable thereby, of an automatic volume control circuit comprising means providing a controlling potential respon sive to variations in received signal strength, a chassis ground connection therefor, means in said connection providing a fixed source of biasing potential, means providing a second fixed source of biasing potential for said controlled amplifier tubes,'the magnitude of which last named potential is in excess of the potential provided bysaid first named source and in opposition thereto by an. amount to provide a condition of maximum sensitivity for said receiving system,v sensitivity control leadsextendingirom said first named source of fixed-biasing potential, and means connected with said leads forvariably connecting said automatic volume control circuit to ground.

8. In a radio receiving system, the combination with automatic volume control means including a diode'signal rectifier adapted to provide automaticvolumeicontrol potentials, of an output resistor'for said rectifier having a negative terminal; an automatic volume control circuit connected therewith,.filter means in said connection including a series high resistance element and by-pass capacitor, means providing a fixed biasing potential in said last named circuit in series with said output resistor connected between the opposite end thereof and ground in said system, a plurality of amplifier tubes controlled by said automatic volume control means, each of said control tubes having a self bias resistor means providing a biasing potential greater than said first named potential by a predetermined amount and in opposition thereto, and remote control means for reducing the elfect of said first named potential in said automatic volume control circuit comprising a circuit control device and circuit leads connected therewith and with said first named potential source.

9. In a radio receiving system, the combination of an interstage amplifier coupling transformer having a primary winding and a secondary Winding, a signal rectifier device including a cathode and an anode, means for connecting said anode with the primary winding to receive signal energy therefrom, an output resistor for said signal rectifier, means providing a common chassis ground connection for said receiving system, a cathode circuit resistor connected between said cathode and said common chassis ground and having a connection at the cathode end thereof with the positive terminal of said output resistor, an automatic volume control circuit having a negative biasing supply lead connected with the negative terminal of said output resistor, filter means in said connection comprising a series resistor and a by-pass capacitor to the chassis ground from said lead, means for applying direct current to said cathode circuit resistor to cause a predetermined potential drop therethrough to effect a predetermined change in sensitivity of said receiving system, a pair of shielded control leads connected with said cathode circuitresistor, and remote control means connected with said leads and operative to connect the positive end of the signal rectifier output resistor to chassis ground to; the exclusion of said cathode circuit resistor, thereby to change the sensitivity of said receiving system through a predetermined range dependent upon said potential.

10. In a radio receiving system, the combination with automatic volume control means including a diode signal rectifier adapted to provide automatic volume control potentials, of an output resistor for said rectifier having a negative terminal, an automatic volume control circuit connected therewith, filter means in'said connection, an amplifier having a cathode circuit ineluding a resistor providing a fixed biasing potential in series with said output resistor and connected with the opposite positive terminal of said output resistor at a point on said cathode circuit resistor which is positive with respect to ground, said ground providing a common return connection for said automatic volume control circuit, a plurality of amplifier tubes connected with said automatic volume control circuit to receive biasing potential therefrom in response to signals and from said fixed biasing potential, each of said control tubes having a self bias resistor means connected between cathode and ground and providing a biasing potential greater than said first fixed biasing potential in opposition thereto and by a predetermined amount sufficient said first named potential source, and means'for shielding said leads including by-pass paths of low alternating current impedance at the terminal of said leads at said receiving system.

LESTER T. FOWLER. 

